Boot partition

Hello, could somebody recommend a way to have a small boot partition on
sparc64?
I have two disks - sd0 and wd0, however the machine can only boot from
sd0. I would like to use sd0 only for booting and have root file system
mounted on wd0a.
What's the best way to tell booting kernel where root file system is
located? Can this be done dynamically or do I need to recompile kernel?